Download Microbenthos, meiobenthos and fiddler crabs

Vol. 32: 259-264, 1986
MARINE ECOLOGY - PROGRESS SERIES
Mar. Ecol. Prog. Ser.
Published September 18
Microbenthos, meiobenthos and fiddler crabs:
trophic interactions in a tropical mangrove
sediment
A. H. Dye & T. A. Lasiak
Department of Zoology, University of Transkei, Private Bag X5092, Umtata, Transkei, Southern Africa
ABSTRACT: Aspects of the feeding ecology of Uca vocans and Uca polita from tropical mangroves
were studied. Gut analysis revealed that microheterotrophs were the major food source for both species
although U. polita also ingested small numbers of microalgae. No evidence was found for the ingestion
of meiobenthos. Based on measurements of feeding rate and mass of feeding pellets produced it is
estimated that the U. vocans population forages 43 % of the sediment surface during a low-tide feeding
period of 2 to 2.5 h. The corresponding figure for U. polita is 22 %. Abundance of meiobenthos
increased 2 to 5-fold when crabs were excluded from the sediment. It is suggested that avoidance by
downward migration and/or competition for food resources may account for this difference.
INTRODUCTION
In marine sediments the benthic components
involved in energy flow are the microbial community,
the meiobenthos and the macro-deposit-feeders.
Recently it has been shown that between 1 and 3 % of
both heterotrophic microbial and autotrophic standing
stock is consumed by meiobenthos (Montagna 1984).
Despite the considerable amount of information on the
ecology of meiobenthos (Swedmark 1964, Jansson
1968, McIntyre 1969, Giere 1975, Ankar & Elmgren
1976, Lasserre 1976) their precise role in benthic communities remains a matter for speculation (Gerlach
1971, 1978, Lee et al. 1974). Several studies have
shown that meiobenthos constitute a food source for
macro-deposit-feeders (Teal 1962, Braber & De Groot
1973, Siebert et al. 1977, Bell & Coull 1978).
Although it has long been suggested that bacteria
may be an important food source for macrobenthos
(Baier 1935, ZoBell & Feltham 1942, Newell 1965,
Odum & De la Cruz 1967) there has been considerable
debate concerning the relative importance of bacteria
and detritus in the diet of deposit-feeders (Kirby-Smith
1975).Studies such as those of Newell (1965),Lopez et
al. (1977) and Lopez & Cheng (1983) provide evidence
for the utilization of the microbial component rather
than detritus per se. On the other hand the work of Teal
(1962) and Adams & Angelovic (1970) provides evidence to the contrary.
@ Inter-ResearchIPrinted in F. R. Germany
Despite the fact that fiddler crabs are dominant
macro-deposit-feeders in many parts of the world little
has been done to elucidate their interactions with the
microbial community (Hoffman et al. 1984). The present study provides data on the relative contribution of
various components of the sediment microbial community to the diet of Uca vocans (Latr.) and Uca polita
(White) from tropical mangroves. Data on the response
of the meiobenthos subsequent to the exclusion of
crabs from the sediment are also presented.
METHODS
Study sites. The studies were camed out on a mudbank at the northern end of Bowling Green Bay close to
the Australian Institute of Marine Science in Queensland (19" 15'S, 147" 30'E). The substratum was characterized by a gradation from muddy sand at LWS to fine
mud at HWS and supported a large population of
fiddler crabs. Uca vocans predominated in the fine
mud whereas U. polita was most abundant in the
muddy sand. Both the sampling and caging experiments were carried out at approximately the mid-tide
level along a 20 m stretch of the mud bank. Semidiurnal tides characterized the area and tidal
amplitude was 1.5 m.
Feeding studies. Crab feeding was studied by comparing unforaged sediment with both fresh feeding
Mar. Ecol. Prog. Ser. 32: 259-264, 1986
260
pellets and the gut contents of fed and starved crabs.
Twenty g of fresh feeding pellets were collected
shortly after the start of a feeding session. After
approximately 1 h of feeding activity the gut contents
of 20 specimens were removed and preserved in 5 O/O
formol-saline. At the same time 7 specimens of each
species were brought back to the laboratory and kept
in 0.45 pm filtered sea water without access to food for
48 h. The water was changed twice during the starvation period.
Meiofauna were counted on 20 X 1 g samples of
sediment and feeding pellets after each sample was
stained with 1 m1 0.5 OO/ Rose Bengal. In each case the
entire sample was stained and counted under a dissecting microscope. Bacteria, Protozoa and algae were
determined on similar samples after first homogenizing the sediment in a Sorval homogenizer at 16000
rpm with 50 m1 of filtered sea water (0.45 pm) as
described by Dye (1979, 1983). Extracted samples were
stained with Acridine Orange and counted by epifluorescence microscopy (Daley & Hobbie 1975).
The feeding intensity of the crabs was determined
from observations of 30 specimens of each species over
60 s periods. Since the actual feeding motions were too
rapid for accurate counting, the number of feeding
pellets produced per min was noted. The mass of the
pellets was determined after approximately 500 had
been collected from each species and oven-dried.
Caging experiments. The fact that Uca vocans and U.
polita tended to occur on different substrata made it
possible to study these species separately. In order to
fit between existing burrows the exclusion cages had
to be smaller than the control cages and were constructed from 10 cm lengths of PVC pipe 20 cm in
diameter, to which a 20 cm high sleeve of 10 mm steel
mesh was fitted. Two such cages were sunk so that
only the mesh protruded above the substratum. A 5 cm
high strip of 1 mm plastic mesh was wrapped around
the side to exclude other macro-deposit-feeders.
As controls, 3 square 0.25 m2 cages were used to
enclose 4 to 6 crabs and their established burrows. The
cages consisted of a 10 cm high skirt of sheet steel
topped with an equal height of 10 mm steel mesh,
which was also used for the roof of the cage. Again the
assembly was sunk to mesh depth and a 5 cm high strip
of 1 mm plastic mesh was wrapped around the sides.
The cages were left in position for 14 d. At the start 3
sets of 10 X 1.6 cm2 cores were taken from each control
cage and 3 sets of 5 cores were taken from the exclusion cages. Samples were taken with a modified 10 m1
plastic synnge. Only the top 1 to 2 mm of sediment was
retained, its fluid nature making it difficult to be more
precise. The procedure was repeated at the end of the
experiment.
RESULTS
Feeding
Feeding pellets from Uca vocans contained 31 % less
bacteria than the sediment but did not differ from
sediment in respect of microalgal content (Table 1 ) .
The guts of freshly fed U. vocans contained bacteria
and detritus but no trace of algae could be found.
There was also no significant difference in the density
of Protozoa between feeding pellets and fresh sediment (t-test, pC0.05) although Protozoa were found in
the gut. There was a significantly lower density of
meiofauna in the feeding pellets compared with sediment (t-test, p<0.05) but no trace of meiofaunal
remains was ever found in guts. Although small
amounts of sediment were found in the guts of starved
crabs residual populations of bacteria and Protozoa
were small (Table 1).
Uca polita feeding pellets had less bacteria, Protozoa
and algae than the fresh sediment (Table 1).There was
a small residual population of bacteria present after
Table 1. Uca vocans and U. polita. Comparison of unforaged sediment with the feeding pellets and gut contents of fed and
starved fiddler crabs. Sed: sediment; FP: feeding pellet; Stvd: Starved
Organism
N
Sed
20
Bacteria
N
X
log g-' ~ S D
Algae
N
X
106 g-' k SD
Protozoa
X lo7 g-l -t SD
Meiofauna
N g" f SD
N
2.46
Uca vocans
FP
Fed
20
20
Stvd
Sed
7
20
f 0.18
1.69
f 0.23
1.47
f 0.79
f 0.36
6.57
f 3.68
6.00
f 3.95
0.00
3.60
+ 0.13
t 1.90
183
t 31
4.27
63
f21
0.68
1.23
t 0.16
+
2.33
Uca polita
Fed
FP
20
20
Stvd
7
+ 0.28
+ 0.63
1.29
0.72
f 0.49
0.00
4.60
k2.14
3.37
k1.54
0.26
f0.07
0.00
0.46
0.06
2.10
i- 0.25
1.35
k 0.60
+ 0.26
0.58
0 00
0
0
f
191
50
2
59
32
0.06
+ 0.06
261
Dye & Lasiak. Flddler crabs in a mangrove sediment
starvation. Table 1 shows that there was some ingestion of microalgae but the number found in the gut was
low compared with that found in sediment. There was
a significant decrease in meiofaunal density (t-test, p <
0.05) in feeding pellets compared with sediment but
there were no melofaunal remains in the gut.
Although more than 4 h was available for feeding
dunng low tide, crabs of both species spent only 2.5 to
3 h actually ingesting material. Females emerged
about an hour before males and retreated into their
burrows about 1 h ahead of the males. The larger crab,
Uca vocans (carapace width 20.46
0.97 mm), produced feeding pellets at a rate of 8.6 -t 0.46 min-' each
weighing 37.5 ? 6.0 mg dry. Based on an observed
foraging depth of 1.0 mm this represents a foraged area
of 214 k 20 cm2 per f e e d n g session (2.5 h). U. polita
(carapace width 14.66 -t 0.88 mm) produced 19.3 -t 1.3
f e e d n g pellets min-l, each with a dry mass of of 7.5 t
0.5 mg. This represents a foraged area of 108.0 k 9.5
cm2 per feeding session. At a density of 20 k 5 crabs
per m2 U. vocans may forage 43 k 4 % and U. polita
may forage 22
2 % of the mud surface. No significant differences in the rate at which feeding pellets are
produced were found between the sexes of either
species (t-test, p < 0.05) and there was no evidence of
coprophagy.
Meiobenthos was present in the surface sediment at
a density of 41 -t 11 animals cm-2 and was numerically
donunated by nematodes (98 %). When crabs were
excluded from the sediment the density of meiobenthos increased 5-fold in the Uca vocans area and up to
3-fold in the area occupied by U. polita (Fig. 1 ) . These
increases were highly significant (t-test, p < 0.001).
+
+
---I
DISCUSSION
The present study indicates that bacteria, and to a
lesser extent Protozoa, are important food sources for
both Uca vocans and U. pohta. Although both species
had detrital particles in their guts it is not known to
what extent, if any, this material is assimilated. The
fact that detrital material was present after 48 h of
starvation is consistent with the conclusions of several
studies that deposit-feeders assimilate mainly the microbial component of the ingested material (Newell
1965, Hargrave 1971, Lopez et al. 1977).
Neither species appears to ingest large amounts of
microalgae despite the fact that the surface of the
sediment was often obviously green and preliminary
observations on surface scrapes indicated considerable
numbers of large pennate diatoms a s well a s
nematodes. It is suggested that the flotation feeding
mechanism (Miller 1961) eliminates large particles,
among which are the majority of microalgae and
meiobenthos. This agrees with the findlngs of Robertson & Newell (1982) on the particle selection abihty of
various Uca species. The large discrepency between
the density of meiobenthos in fresh sediment and in
feeding pellets may stem from a difference in sampling. The fluid nature of the sediment and the frequent presence of small grit particles made it difficult
to ensure that only the upper 1 mm was retained from
every core. Since Uca spp. forage mainly the upper 1
mm the density of meiofauna available may b e somewhat lower than indicated although nematodes are
shll abundant.
The increase in abundance of meiofauna in the
UNGRAZED
I
U. polita
U. vocans
GRAZED
m
UNGRAZED
m
GRAZED
T 1 4
Fig. 1. Effect of fiddler crab exclusion on the abundance of meiobenthos in surface sediments. Ungrazed N
N = 30 ( x 3 ) .To: start of experiment; T,,: after 14 d exclusion
=
15 ( x 2 ) , grazed
262
Mar Ecol. Prog. SE
exclusion cages may arise in a number of ways: by
passive deposition d u e to tidal currents, by growth and
reproduction leading to a n increase in biomass, or by
upward migration from deeper layers. Tidal suspension a n d transport of meiofauna has been reported by a
number of workers. Although various mechanisms
have b e e n proposed, t h e work of Hagerman & h e g e r
(1981), Siebert (1981) and Palmer & Gust (1985) supports the idea of passive transport resulting from
mechanical scouring. This process could presumably
b e enhanced by mechanical disturbance of sediment
such a s is caused by Uca spp. Any mechanical obstruction such as a cage will affect the flow characteristics of
water close to the sediment surface a n d thus influence
the processes of suspension a n d deposition. No significant differences in the abundance of meiofauna
were detected in the grazed cages over the 14 d period
(t-test, p < 0.05) and the effect appears to be minimal
in this case. No obvious deposition of material was
seen in t h e exclusion cages a n d it is difficult to estimate to what extent, if any, passive deposition contributed to the increase in meiofaunal abundance.
Palmer & Gust (1985) found a maximum of 0.7 O/O of
sediment meiofauna suspended in the water column
and their mean was considerably lower than this. It
seems unlikely that deposition alone could account for
the substantial increase in meiofaunal abundance
reported here.
Estimates of meiofaunal productivity indicate a turnover of between 5 and 10 yr-' depending on conditions
(McIntyre 1969, Gerlach 1971, Arlt 1973). The duration
of the present experiment was only 14 d. At the higher
turnover this means a n Increase of about 40 O/O which is
far short of the actual observed increase. This also does
not take into account mortality during the experimental period.
Vertical migration of meiofauna is well documented,
particularly in sandy areas, a n d is usually associated
with desiccation (Renaud-Debyser 1963, Boaden 1968,
Harris 1972, McLachlan et al. 1977, Dye 1978). The
decrease in stability of the surface sediments caused
by Uca spp. may force the meiofauna to migrate downwards. When this effect is removed, a s in the exclusion
cages, meiofauna return to the surface once again.
There may even be a cyclic migration of meiofauna
tuned to the foraging activity of the crabs.
From the number a n d mass of pellets produced ~t is
evident that these crabs can process a significant area
of substratum during a low tide period. Although the
crabs feed only during diurnal low tide (Salmon 1984)
there are several days a month when low tide occurs
twice during daylight. With the possible ingestion of
u p to 30 "10 of the standing stock of bacteria it can b e
expected that foraging by fiddler crabs wd1 have a
significant effect on edaphic microbial populations.
In a study of the grazing effects of the gastropod
Nassarius obsoletus, Pace et al. (1979) found a significant decrease in sediment ATP after 12 d of grazing,
when compared with ungrazed plots. Connor & Teal
(1982) found that intense grazing by Ilyanassa obsoleta
reduced algal standing stock and depressed photosynthesis and respiration. Although a considerable
amount is known about the feeding mechanisms and
behaviour of fiddler crabs there is little information on
their effect on the sediment microbial community. A
recent study by Hoffman e t al. (1984) on the effects of
foraging by Uca pugnax on meiofauna yielded similar
results to those of the present study. An order of magnitude increase in the abundance of nematodes in
surface sedlments accompanied total removal of crabs
a n d partial removal resulted in almost a doubling of
meiofauna density. While these data clearly indicate
that the presence of fiddler crabs in some way depresses meiofaunal abundance there is Little evidence in
favour of predation. Hoffman e t al. (1984) do not present data on gut contents a n d the possibility that the
effect is secondary cannot b e discounted. If fiddler
crabs and meiofauna were, for example, competing for
the same food resource (detrituslbacteria) then the
removal of the crabs should favour meiofauna a n d
result in a n increase in its abundance. Further study is
required to resolve the relative importance of avoidance and competition in this regard.
As noted by Tenore (1983) the relative Importance of
microbes in the nutrition of deposit-feeders depends
on the quality of carbon and nitrogen sources in the
sediment. For examp1.e seaweed detntus a n d diatoms
may be consumed directly while more refractory material such as mangrove and salt marsh detritus
requires conversion to microbial carbon and nitrogen
prior to ingestion. In the present case most of the
diatoms appear to be excluded by the feeding mechanism. The major source of organic material is mangrove
detritus and hence microbes can be expected to constitute a n important source of nutrition for fiddler crabs.
Whether bacteria alone can satisfy the nutritional
requirements of deposit-feeder is open to question.
Tunnicliife & Risk (1977) concluded that low microbial
production forces the deposit-feeding bivalve Macorna
balthica to switch to suspension feeding when submerged, to supplement its diet. h t t l e is known about
the nutritional requirements of tropical flddler crabs
but Hargrave (1971) has estimated that ingestion of
10 O/O of the daily microbial production is sufficient to
meet the food requirements of the amphlpod Hyallela
azteca. Microbial production is usually high in tropical
sediments and, as suggested by Newel1 & Field (1983)
for benthic communities, ingestion of a relatively small
proportion of this production may well be sufficient for
consumers.
Dye & Lasiak: Fiddler crabs in a mangrove sediment
Acknowledgements. The authors are indebted to the Council
for Scientific and Industrial Research (South Africa) and the
University of Transkei for their financial assistance. The
Australian Institute of Marine Science is thanked for its considerable material assistance. Dr. A. Robertson is thanked for
constructive comments during the course of the work, and Dr
W. Emmerson for commenting on the manuscript
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Thls article was presented by Dr. J. S. Bunt: it was accepted for printing on June 30. 1986